1. Field of the Invention
The present invention relates to a recording apparatus for image recording such as a laser beam printer.
2. Related Background Art
In a conventional apparatus for image formation on a photosensitive drum by light beam scanning, for example with a rotary polygonal mirror, such as a laser beam printer, a horizontal synchronization signal is generated from a detection signal indicating that the scanning light beam has reached a scan starting end of the photosensitive drum. For example, in a printer in which a light beam scans a photosensitive drum from left to right, a photodetector is provided close to the left-hand end of the drum, and a horizontal synchronization signal is generated by delaying a detection signal, obtained from the photodetector when the light beam passes, by a determined perior in such a manner that the synchronization signal coincides with the arrival of the light beam at the left-hand end of a recording sheet. Such method is satisfactory if the recording sheet, for receiving the image from the drum after the development of a latent image, is always aligned with to a left-end position regardless of the width of sheet, but the determined period of delay has to be altered if the recording sheet is supplied in such a manner that the center thereof is aligned with to the central axis of a scanning system, or the center of the photosensitive drum, for example in order to utilize better optical characteristics of the central portion of the scanning system. Such alteration can be achieved by counting pixel clock signals corresponding to the pixels to be recorded or other clock signals having a multiple frequency, from the moment the light beam passes the photodetector, and generating the horizontal synchronization signal at a suitable count which is variable according to the size of the recording sheet.
In a conventional printer in which a printer unit is separate from a control unit, pixel clock signals are generated in the printer unit and are sent, together with the horizontal synchronization signal, to the control unit, which sends image signals to the printer unit in synchronization with the pixel clock signals, from the reception of the horizontal synchronization signal. In such structure, if the cable between the printer unit and the control unit is long, the image signals received by the printer unit show a considerable phase delay with respect to the pixel clock signals generated in the printer and often cannot be synchronized therewith, thus giving rise to a distortion in image or a loss of data. Also in the above-explained conventional printer, if the frequency of the pixel clock signals is changed for modifying the image density or for enlarging or reducing the recorded image, the above-mentioned count for generating the horizontal synchronization signal must also to be modified.